In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, for future generations of mobile communications systems frequency bands at many different carrier frequencies could be needed. For example, low such frequency bands could be needed to achieve sufficient network coverage for terminal devices and higher frequency bands (e.g. at millimeter wavelengths (mmW), i.e. near and above 30 GHz) could be needed to reach required network capacity. In general terms, at high frequencies the propagation properties of the radio channel are more challenging and beamforming both at the network node at the network side and at the terminal devices at the user side might be required to reach a sufficient link budget.
In general terms, the use of beamforming could imply that the terminal devices will be not only operatively connected to the network node via a beam but also performs a handover between (narrow) beams instead of between network nodes of different cells, or between transmission and reception points (TRPs) of one and the same network node. At higher frequency bands high-gain beamforming with narrow beams could be used due to more challenging radio propagation properties than at lower frequency bands. Each beam will only be optimal within a small area and the link budget outside the optimal beam will deteriorate quickly. Hence, frequent and fast beam switching is needed to maintain high performance. This is hereinafter referred to as beam management. One purpose of so-called beam management is thus for the network node to keep track of its served terminal devices with narrow beams (as used at the TRP of the network node and/or at the terminal devices) in order to increase coverage and throughput.
A suitable TRP transmission beam for each terminal device is expected to be discovered by the terminal devices and monitored by the network using measurements on downlink reference signals used for beam management, such as channel state information reference signal (CSI-RS), as performed by the terminal devices. The downlink reference signals could, for beam management purposes, be transmitted periodically, semi-persistently or aperiodically (e.g. being event triggered) and they can be either shared between multiple terminal devices or be specific for a particular terminal device. In order to find a suitable transmission beam to each terminal device the TRP transmits reference signals in different TRP transmission beams on which the terminal devices performs measurements, such as measurements of reference signal received power (RSRP), and reports back the Nbest TRP transmission beams (where N can be configured by the network).
The terminal devices and/or the TRP of the network node could implement beamforming by means of analog beamforming, digital beamforming, or hybrid beamforming. Each implementation has its advantages and disadvantages. A digital beamforming implementation is the most flexible implementation of the three but also the costliest due to the large number of required radio chains and baseband chains. An analog beamforming implementation is the least flexible but cheaper to manufacture due to a reduced number of radio chains and baseband chains compared to the digital beamforming implementation. A hybrid beamforming implementation is a compromise between the analog and the digital beamforming implementations. As the skilled person understands, depending on cost and performance requirements of different terminal devices, different implementations will be needed.
One drawback with an analog beamforming implementation is that the TRP only can transmit or receive in one beam at a time (assuming one antenna array, and that on and the same beam is used for all polarizations, which typically is the case in order to counteract dropped signal strength due to polarization mismatching). This reduces the possibility to serve multiple terminal devices simultaneously (by e.g. using frequency multiplexing).
In order to increase the possibility to co-schedule terminal devices simultaneously in the same TRP transmission beam (when analog beamforming is used), as much information about which TRP transmission beams that are suitable for each terminal device is desirable. However, attaining this information at the TRP for all terminal devices in the network would require unnecessary high overhead signalling.
Hence, there is still a need for improved mechanisms for co-scheduling terminal devices.